1. Field of the Invention
The invention relates generally to flexible laminates and more particularly to extruded thermoplastic adhesive laminates, wherein the laminate is particularly suitable for forming collapsible structures such as fuel tanks, water tanks, inflatable boats and other solid/liquid storage tanks.
2. Prior Art
Traditionally, collapsible tanks are constructed of panels that are assembled into the desired shape either by compression press or hot-air heating and pressure or RF welding. Flexible laminates are made by applying a solvent based adhesive on a nylon or polyester or Kevlar fabric, which is then extrusion coated with a thermoplastic polyurethane (TPU) or a TPU alloy onto the adhesive side of the coated fabric. In the current art the solvent based adhesive typically has a curing additive which cross-links the adhesive, thereby securely anchoring the extruded TPU to the fabric. It is recognized that the collapsible structures can undergo rather severe oxidative and hydrolytic conditions. As an example, in a water tank, the water tank can be exposed to the external weather elements for an indefinite, and potentially extended, period of time. Also, the collapsible structures used as fuel tanks need to be resistant to the solvating effects of fuel and, therefore, must be virtually impervious and non-swelling in the presence of the fuel. In both cases a relatively high-degree of cross-linking is desired to minimize the deleterious effects of fuel, oxygen, and water. To a certain extent the TPU is selected so that it is inherently resistant to aging, hydrolysis, oxidation and swelling. Typically, the adhesive laminate, as well as the assembled tank or other collapsible structure needs to retain bond strength and seam strength under severe conditions. The minimum acceptable bond performance is 25 lbs/in, following immersion in water and/or fuel at 160° F. for six weeks.
While conventional solvent based adhesives meet the performance requirements for collapsible structures there are various process-related problems. The major concern is the generation of air-borne emissions, typically as defined by the EPA as VOCs (e.g. volatile organic compounds). Normally, VOCs are captured by solvent recovery systems, or are disintegrated using catalytic converters (a.k.a. thermal oxidizer). Solvents have the additional problem that in addition to their cost of use and cost of capture they are flammable, and therefore represent an on-going threat of fire. What is desired is a laminating adhesive suitable for collapsible structures that does not utilize a solvent-based adhesive.